Mesenchymal stem cells (MSCs) are fibroblast-like cells that reside within virtually all tissues of a postnatal individual. Within bone marrow (BM), from which they were first isolated more than 40 years ago, MSCs provide the stromal support tissue for hematopoietic stem cells. In addition, MSCs can differentiate into cells of the mesenchymal lineage, such as osteoblasts, adipocytes, and chondrocytes. MSCs can interact with immune cells at various activation stages in vitro and modulate the immune cells' activity through a variety of mechanisms. Tse et al., Transplantation 75:389-397 (2003); Le Blanc et al., Exp. Hematol. 31:890-896 (2003); Rasmusson et al., Transplantation 76:1208-1213 (2003); Di Ianni et al., Exp. Hematol. 36:309-318 (2008).
Because of their regenerative and immunomodulatory properties, MSCs are implicated in the treatment of a great number of disorders associated with aberrant immune function and inflammation. Le Blanc et al., Cytotherapy. 7:36-45 (2005); Jones et al., Exp. Hematol. 36:733-741 (2008); Javazon et al., Exp. Hematol. 32:414-425 (2004). Ongoing phase I-III clinical trials examine the use of MSCs as possible treatment of a variety of disorders. For example, MSCs made from allogeneic third party BM are effective in treating acute graft versus host disease after allogeneic hematopoietic stem cell transplantation. Le Blanc et al., Lancet. 371:1579-1586 (2008). Also, allogeneic or autologous MSCs from BM or other tissues are being examined as treatment for a wide variety of conditions, such as myocardial infarction, amyotrophic lateral sclerosis, stroke, Crohn's disease, diabetes mellitus, and refractory non-healing wounds. Yoshikawa et al., Plast. Reconstr. Surg., 121:860-877 (2008). A major obstacle to using BM-derived MSCs in a clinical setting is the time, usually several weeks, needed to culture and expand a sufficient number of MSCs for application.
While the interaction of MSCs with T-lymphocytes, B-lymphocytes, natural killer cells, and dendritic cells have been extensively studied, little is known about the interaction of MSCs with other cells of the monocytic lineage, specifically macrophages. Macrophages are widely distributed in many different tissues and are a key component of innate immunity. Like other hematopoietic cells, macrophages originate from a bone marrow precursor cell that first gives rise to monocytes. Monocytes circulate in the peripheral blood and can give rise to macrophages after extravasating from the blood stream into the surrounding tissue, either to replace long-lived tissue macrophages or in response to injury. Gordon, European J Immunol. 37 Suppl 1:S9-17 (2007).
Studies with murine macrophages and MSCs demonstrated that MSC-conditioned media inhibits the capacity of an activated murine leukemic monocyte/macrophage cell line (RAW-264.7) to secrete TNF-α. Ortiz et al., PNAS 104:11002-11007 (2007). Further, LPS-stimulated macrophages produced more IL-10 when co-cultured with murine MSCs. Significantly more of the macrophages isolated from septic mice treated with MSCs contained intracellular IL-10. Nemeth et al., Nature Med. 15:42-49 (2009). However, these studies were conducted using murine cells, which differ in many regards from human MSCs or macrophages. Peister et al., Blood 103:1662-1668 (2004); Schneemann and Schoeden, J Leukocyte Biol. 81:579 (2007), each of which is incorporated herein by reference as if set forth in its entirety. The inventors were first to study human macrophage-MSC interactions.
Although various categories of classification have been proposed, macrophages are typically divided into classically-activated macrophages (M1 macrophages) and alternatively-activated macrophages (M2 macrophages). Martinez et al., Annu. Rev. Immunol. 27:451-483 (2009). Generally, M1 macrophages exhibit potent anti-microbial properties, reminiscent of type 1 T-helper lymphocyte (Th1) responses. In contrast, M2 macrophages promote type 2 T-helper lymphocyte (Th2)-like responses, secrete less pro-inflammatory cytokines, and assist resolution of inflammation by trophic factor synthesis and phagocytosis. Mosser et al., Nature Rev. 8:958-969 (2008). M2 macrophages can be further divided into three distinct subclasses, i.e., M2a, M2b, and M2c, defined by specific cytokine profiles. Mantovani et al., Trends Immunol. 25:677-686 (2004). While M2 macrophages are generally characterized by low production of pro-inflammatory cytokines, such as IL-12, and high production of anti-inflammatory cytokines such as IL-10, M2b macrophages retain high levels of inflammatory cytokine production, such as TNF-α and IL-6. Mosser, J. Leukocyte Biol. 73:209-212 (2003).
Macrophages can be polarized by their microenvironment to assume different phenotypes associated with different stages of inflammation and healing. Stout et al., J. Immunol. 175:342-349 (2005). Certain macrophages are indispensible for wound healing. They participate in the early stages of cell recruitment and of tissue defense, as well as the later stages of tissue homeostasis and repair. Pollard, Nature Rev. 9:259-270 (2009). Macrophages derived from peripheral blood monocytes have been used to treat refractory ulcers. Danon et al., Exp. Gerontol. 32:633-641 (1997); Zuloff-Shani et al., Transfus. Apher. Sci. 30:163-167 (2004), each of which is incorporated herein by reference as if set forth in its entirety.